In the mammalian visual system, signals are relayed from the retina to the primary visual cortex (V1) via the lateral geniculate nucleus (LGN). Over the past 30 years, three main cell types with very different properties have been identified in the LGN: parvocellular, magnocellular, and koniocellular. How do V1 cells interpret spike trains from different types of LGN cells? We propose experiments to explore the synaptic physiology of connections from LGN to V1. In one set of experiments, we will record action potentials from many individual cells in the squirrel LGN using tetrodes, and record from single units in V1 using conventional electrodes. We will fully characterize the physiological properties of the LGN and V1 cells in response to natural input, and look for evidence of monosynaptic connections using cross-correlation techniques. We will see if LGN cells only contact V1 cells with similar properties or if the connections are diffuse. In a second line of experiments, we will record many LGN cells using tetrodes and will make whole cell (intracellular) recordings in V1. We will fully characterize the LGN and V1 cells, and, using spike-triggered averaging, we will look for monosynaptic connections between the LGN and V1 cells. Finally, we will characterize the synaptic dynamics of these connections. Understanding the transfer of information from LGN to V1 will aid in understanding developmental diseases such as amblyopia, and understanding how cortical cells function will aid in understanding many diseases or injuries of the cortex such as Alzheimer's and stroke, and this knowledge will inspire treatments for these ailments.